3GPP design, specify and standardise technologies for mobile (cellular) wireless communications s. Specifically 3GPP produces a series of technical reports (TR) and technical specifications (TS) that define 3GPP technologies. The focus of 3GPP is currently the specification of standards beyond the 3rd Generation (3G) of mobile networks, and in particular an Evolved Packet System (EPS) offering enhancements over 3G networks, in particular higher data rates. The set of specifications for the EPS comprises two work items: Systems Architecture Evolution (SAE, concerning the core network) and LTE concerning the air interface. The first set of EPS specifications were released as 3GPP Release 8 in December 2008. Despite LTE strictly referring only to the air interface, LTE is commonly used to refer to the whole of the EPS, including by 3GPP themselves. LTE is used in this sense in the remainder of this specification. LTE enhancements, including LTE Advanced offering still higher data rates compared to LTE, have followed up to and including 3GPP Release 11 in September 2012. LTE Advanced is considered to be a 4th generation (4G) mobile communication system by the International Telecommunication Union (ITU). LTE is an evolution of the 3GPP 3G Universal Mobile Telecommunication System (UMTS) and shares certain high level components and protocols with UMTS. A key difference is that whereas UMTS allows for both PS and Circuit Switched (CS) data transfer, with CS being used for voice calls, LTE is a wholly PS system for the delivery of packet data, regardless of the content of the data packets.
As noted above, the present invention is not limited to LTE. However, particular embodiments of the present invention may be implemented within an LTE mobile network and detailed descriptions of embodiments of the invention are presented in the context of an LTE network. Therefore, an overview of an LTE network is shown in FIG. 1. The LTE system comprises three high level components: at least one UE 102, an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) 104 and an Evolved Packet Core (EPC) 106. The EPC 106 communicates with Packet Data Networks (PDNs) and servers 108 in the outside world. FIG. 1 shows the key component parts of the EPC 106. It will be appreciated that FIG. 1 is a simplification and a typical implementation of LTE will include further components. In FIG. 1 interfaces between different parts of the LTE system are shown. The double ended arrow indicates the air interface between the UE 102 and the E-UTRAN 104. For the remaining interfaces media is represented by solid lines and signalling is represented by dashed lines.
The UE 102 comprises a communication device, referred to as Mobile Equipment (ME) and a Universal Integrated Circuit Card (UICC) which is a smart card, commonly referred to as the Subscriber Identity Module (SIM) card. The UICC stores identification information which uniquely identifies the UE user.
The E-UTRAN 104 comprises a single type of component: the evolved Node B (eNB) which is responsible for handling radio communications between the UE 102 and the EPC 106 across the air interface. An eNB is a base station that controls UEs 102 in one or more cell. Typically there is a plurality of eNBs within an LTE system, and the eNBs comprising the E-UTRAN are connected to the EPC 106.
Key components of the EPC 106 are shown in FIG. 1. It will be appreciated that in an LTE network there may be more than one of each component according to the number of UEs 102, the geographical area of the network and the volume of data to be transported across the network. Data traffic is passed between each eNB and a corresponding Serving Gateway (S-GW) 110 which routes data between the eNB and a PDN Gateway (P-GW) 112. The P-GW 112 is responsible for connecting a UE to one or more servers or PDNs 108 in the outside world. The Mobility Management Entity (MME) 114 controls the high-level operation of the UE 102 through signalling messages exchanged with the UE 102 through the E-UTRAN 104. Each UE is registered with a single MME. There is no direct signalling pathway between the MME 114 and the UE 102 (communication with the UE 102 being across the air interface via the E-UTRAN 104). The air interface can be considered to be split into two levels: the Access Stratum (AS) and the Non Access Stratum (NAS). Signalling messages between the MME 114 and the UE 102 are transported via the NAS protocol. Signalling messages between the MME 114 and the UE 102 comprise EPS Session Management (ESM) protocol messages controlling the flow of data from the UE to the outside world and EPS Mobility Management (EMM) protocol messages controlling the rerouting of signalling and data flows when the UE 102 moves between eNBs within the E-UTRAN. The MME 114 exchanges signalling traffic with the S-GW 110 to assist with routing data traffic. The MME 114 also communicates with a Home Subscriber Server (HSS) 116 which stores information about users registered with the network.
An LTE system transports data from one part of the system to another via bearers. The most important bearer from the perspective of an end user is the EPS bearer between a UE 102 and a P-GW 112. The EPS bearers are responsible for transporting data between the UE 102 and servers and PDNs outside of the EPC 106. In the remainder of this specification the term “bearer” will be used generically (including EPS bearers) to refer to any bearer between the UE 102 and another part of the LTE network, and in particular part of the EPC.
A bearer may be considered to be a logical or virtual connection between two end points. A bearer may also be considered to be a data pipe between two such end points. Each bearer may be defined in terms of minimum Quality of Service (QoS), minimum data rate, maximum error rate and maximum delay. In an LTE network, for each UE 102 when the UE first registers with the network, it either connects to a PDN for the default Access Point Name (APN), which is configured in the HSS, or it connects to a PDN for an APN provided by the UE at time of Attach. On establishing a standalone PDN connection, a default EPS bearer is established between the UE and a selected P-GW 112. Additional PDN connections may later be established between the UE 102 and alternative P-GWs 1125, which results in the establishment of further default EPS bearers for these PDN connections. Dedicated bearers may be established by the EPC between the UE 102 and a P-GW 112 for which there exists a default bearer, for instance in order to establish a data flow with a higher QoS or minimum bit rate.
A UE 102 requests a new bearer for a new data communication session if the required service cannot be provided upon the default bearer. This includes when establishing a new voice call. An LTE network, and in particular an MME 114 responsible for that UE 102, can only support a limited number of bearers per UE 102. This maximum number of bearers per UE 102 is defined by the LTE specifications based on a protocol limitation defined in sub-clause 11.2.3.15 of 3GPP Technical Specification 24.007 version11.0.0 Release 11, “Group Core Network and Terminals; Mobile Radio Interface Signalling Layer 3; General Aspects” available from http://www.3gpp.org. The maximum number of bearers in theory cannot exceed 11. For a General Packet Radio Service (GPRS) network, the corresponding limit on Packet Data Protocol (PDP) contexts is defined in sub-clause 10.5.6.2 of 3GPP Technical Specification 24.008 version 11.4.0 Release 11, “Digital cellular telecommunications system (Phase 2+); Universal Mobile Telecommunications System (UMTS); LTE; Mobile radio interface Layer 3 specification; Core network protocols; Stage 3” available from http://www.3gpp.org. However, in practice this limit can vary widely between different LTE implementations. However, as defined by the existing LTE specifications, a UE 102 knows only the number of active bearers that it has established with the network, and not the maximum number of available bearers. Referring now to FIG. 2, this illustrates a situation in which the maximum number of bearers for a UE 102 has already been reached, and the UE 102 requests a further bearer. In particular, FIG. 2 illustrates a situation in which the UE 102 requests a further bearer in order to allow a user to make an emergency voice call.
FIG. 2 shows signalling messages exchanged between a UE 102 and the associated MME 114 with which the UE 102 is registered. As noted at 202, the UE has already established a default bearer for a connection to an external PDN (the internet) and a default bearer for a connection to an IP Multimedia Subsystem (IMS). IMS is a separate 3GPP specified network communicating with the EPC for handling real time IP multimedia services for example Voice over IP (VoIP), which is used by LTE for voice calls. As noted at 204 the MME supports a maximum of two bearers per UE. It will be appreciated that in an operational LTE network the maximum number of bearers may be significantly higher. The UE is not aware that the MME supports a maximum of 2 bearers, and that the maximum has already been reached.
As noted at 206, the UE and the MME are initially in an EMM-IDLE state. This represents a state in which there is no NAS signalling connection between the UE and the network. It will be appreciated that in an alternative scenario the UE may initially be in the EMM-connected state (that is, in a state in which there is a NAS signalling connection between the UE and the network).
At 208 the user of the UE wishes to make an emergency voice call and takes the necessary action at the UE to cause the UE 102 to request a further bearer. At 210 the UE sends a Service Request message to the MME. The service request message is an EMM message forming part of the service request procedure which takes place when the UE is in the EMM-IDLE state but wishes to communicate with the network. The service request message requests that the MME moves the UE to the EMM-CONNECTED state. At 212 the UE then sends a PDN Connectivity Request message to the MME. The PDN Connectivity Request message is an ESM message which asks the network to establish a bearer for a connection to an emergency PDN. Note that if the UE was already in EMM-CONNECTED state, then step 210 is omitted.
At 214 the MME responds with a PDN Connectivity Reject message including an error code indicating that the maximum number of bearers supported by the MME for that UE has already been reached (error code #65). Error code #65 is defined as the max number of EPS bearers per Public Land Mobile Network (PLMN). This error code is specified in section 6.5.1.4 of 3GPP Technical Specification 24.301 version 11.4.0 Release 11, “Universal Mobile Telecommunications System (UMTS); LTE; Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3” available from http://www.3gpp.org. Section 6.5.1.4 of TS 24.301 specifies ESM error codes in the event that a PDN Connectivity Request cannot be accepted by the network. The UE can then determine the network's maximum number of EPS bearers. This can be stored at the UE, though will be released if the UE is switched off, or the USIM is erased. This is the first time at which the UE becomes aware of the maximum number of bearers supported by the MME.
In order to make the emergency call, the UE performs explicit deactivation of one of the established PDN connections. Specifically, the UE sends a PDN Disconnect Request message 216 to the MME. The MME then sends a Deactivate EPS Bearer Context Request message 218 from, to which the UE responds with a Deactivate EPS Bearer Context Accept message 220. Now that there is one less bearer than the maximum available number of bearers, at 222 the UE again sends the PDN Connectivity Request message requesting a bearer for a connection to an emergency PDN. This time, at 224 the MME responds with an Activate Default EPS Bearer Context Request message which includes the EPS bearer identity for the newly established EPS bearer, together with other information, for example a Quality of Service (QoS) measure and an IP address associated with the bearer which may be needed by the UE to make use of the bearer. At 226 the UE returns an Activate Default EPS Bearer Context Accept message which acknowledges receipt of the new bearer information. Messages 222 to 226 are ESM messages. At 228 the emergency PDN connection has been established and the emergency voice call can take place.
It will be appreciated that FIG. 2 is a simplification of signalling messages sent between the UE and the MME, and in particular the encapsulation of EMM and ESM messages within other lower layer protocols between the UE and the E-UTRAN, or between the E-UTRAN and the MME, has been omitted.
The PDN Connectivity Request message sent from the UE to the MME at 212 identifies an Access Point Name (APN) and PDN type for the requested bearer, together with further Information Elements (IEs) specifying information specific to the requested PDN connection. Sections 6.5.1.4A and 6.5.1.6 of 3GPP TS 24.301 note that as an exception even if the maximum number of bearers has already been established for the UE, if a further PDN Connectivity Request message is received with the same combination of APN and PDN type as an existing, non-emergency PDN connection then the further PDN Connectivity Request message can be accepted under two limited circumstances.
The first circumstance is if all of the IEs in the further PDN Connectivity Request message exactly match those within a previous PDN Connectivity Request message and the MME has not received the Activate Default EPS Bearer Context Accept message 226 from the UE. This relates to a situation in which an Activate Default EPS Bearer Context Request message 224 sent by the MME in response to the earlier PDN Connectivity Request message was not received by the UE, such that the UE has resent the PDN Connectivity Request message. This is a limited error handling exception. Clearly this is of no benefit if the request for a further bearer is in order to make an emergency call (and not to retry a non-emergency call).
The second circumstance is if the IEs do not match, and the APN does not support multiple PDN connections. The MME may optionally either deactivate the existing EPS bearer context for the earlier PDN connection locally without notification to the UE and proceed with the requested PDN connectivity procedure, or the MME may respond with error code #55 “multiple PDN connections for a given APN not allowed”. This second circumstance relates to a situation in which the UE has an already established EPS bearer for a particular PDN connection and requests a similar EPS bearer with different operating parameters, for instance QoS or data rate. This cannot relate to an emergency call scenario as the APN is never included in the PDN connectivity request when establishing an emergency PDN connection. The emergency PDN connection is determined through configuration at the MME.
It will be appreciated that while the signalling illustrated in FIG. 2 allows a bearer to be requested even if the maximum number of bearers has already been allocated, this is at the expense of unnecessary delay due to the UE having no knowledge of the maximum number of available bearers. Specifically, messages 212 and 214 are sent in order to advise the UE that the maximum number of bearers has already been reached. This results in a delay at the time of requesting the new bearer, largely due to the inherent delay for each signal transmitted across the air interface to and from the E-UTRAN. There has been significant effort to minimise all sources of delay within LTE systems, particularly where real time services including voice calls are concerned. However, delays persist if the maximum number of bearers has already been established. It will be appreciated that the source of delay described above in connection with FIG. 2 applies to all bearer requests. However, establishing an emergency call is particularly time sensitive, especially if a delay is sufficiently long that it is perceptible to the end user. Therefore, throughout the remainder of the present specification the example of establishing an emergency call will used.
As noted above, the present invention is broadly applicable to PS systems, and is not limited to LTE. Specifically, the present invention is applicable in any PS system in which connections from a mobile device to a server, network component or PDN comprise a logical or virtual connection or data pipe, and the maximum number of such connections is limited. For instance in 2nd generation (2G)/3G networks based on Global System for Mobile Communications (GSM) or UMTS, in the PS domain the equivalent of an EPS bearer is a PDP context. PDP contexts appear generally the same as EPS bearers from the perspective of the Mobile Station (MS, the equivalent of an LTE UE). One difference is that whereas a default EPS bearer is requested by the MS when registering with the EPC, a PDP context is only requested when the MS wishes to communicate with a server or a PDN in the outside world.
The number of PDP contexts that can be established for a single UE is limited, and not known to the MS. Section 6.1.3.0 of 3GPP TS 24.008 specifies how the maximum number of PDP contexts for an MS is determined. Similarly to the situation illustrated for LTE in FIG. 2, upon receipt of an Activate PDP Context Request Message the network may respond with an activate PDP context reject message with error code # “maximum number of PDP contexts reached”.
Similarly to the exceptions described above in connection with FIG. 2, section 6.1.3.1.5 part (c) of 3GPP TS 24.008 notes that if an MS initiates a PDP context activation request for an already activated PDP context and all of the details of the request match exactly then the existing activated PDP context is deactivated locally within the network and the network proceeds with the requested PDP context activation. This situation relates to a limited error handling situation in which the MS did not receive the response to the original Activate PDP Context Request message. Alternatively, if the details of the new Activate PDP Context Request Message do not match an existing activated PDP context, but the Network layer Service Access Point Identifier (NSAPI) does match that of an existing PDP context then the network will deactivate the existing PDP context locally within the network and the network proceeds with the requested PDP context activation. Clearly the requirement for the NSAPI to match means that this exception only applies in the event the MS wishes to reinitialise the same PDP context for a unique data session, and is of not benefit in the event that the MS wishes to open a new data session. In the majority of instances, if a further Activate PDP Context Request message is received when the maximum number of PDP contexts has been reached the request causes error code #65 to be returned. Furthermore, these exceptions do not apply to a new Activate PDP Context Request Message with request type “emergency” and there is already a PDN connection for emergency bearer services existing. The trigger for the UE to reuse the same NSAPI and different combination of APN, PDP type and PDP address can arise when the UE itself has reached its maximum number of bearers or when the UE knows that the network has reached its maximum number of bearers. However, this does not apply when the network has reached the maximum number of bearers and the UE does not know that (and the UE has not reached its maximum). Thus, the UE has no cause to reuse an existing NSAPI.
The term “bearer” is used in connection with the present invention to refer to any logical or virtual connections between a UE and a component of a mobile communications network or an external server or PDN, and should not be considered to refer only to the particular forms of bearers defined in connection with LTE, despite embodiments of the present invention being described in detail in connection with LTE.